It really depends on your approach. If you say something like: "Hi, I'm Thomas" pay them a compliment based on something you like about them (not how attractive they are) and ask them whether they fancy getting a tea or coffee at some point this week.
The reason I said pay them a compliment is because it's a postive statement and a nice opener ir you use it correctly. Good things to compliment them on could be something they have clearly made an effort with, like if you notice that they have put together a really nice outfit that is something that obviously took a long time. If they have something like an instrument or portfolio with them you could mention that you noticed their portfolio and clearly they are a creative person which you think is cool so blah blah blah.
If you say something like "Hi, I'm Thomas, you're really pretty so I was wondering if..." that can come across a little odd to a lot of people as that implies you spoke to them 100% because of their looks, not because you thought they had a really cool look or found something interesting about them.
Won't work for all but might be much more effective than what it would be like if you were to come across as a creep who went out of his way to talk to a random girl on the street because her boobs looked good in that shirt...